1. Field of Invention
This invention generally relates to the field of mobile and fixed wireless communications systems. More particularly the invention pertains to a methodology for performing radio frequency coverage surveys based on actual measurements of radio signal strengths, as propagated over the path from a test mobile station transmitter to one or more test base station receiver points (sites).
2. Description of Prior Art
In designing and implementing new wireless systems for radio communication, or in expanding existent systems, suitable methods are used to select optimal placements of new base station sites that provide for efficient use of the radio frequency spectrum in the given geographical area.
This optimal placement of the base station sites is based on dividing the geographical area under consideration into cells of radio coverage. Each of these cells is served primarily by one base station site operating in the allocated spectrum for that area.
A mobile device traveling through the geographical area can communicate via the base station serving the cell within which it is currently located. This will be the base station offering the best signal strength to the mobile device at that moment.
The cell used will change as the mobile travels through the geographical area. This change will occur in most cases due to a process called “hand off” based on which base station is offering the best signal to the mobile and visa versa. The term best signal here is based on multiple criteria, the most important of which, is in most cases defined in terms of the signal strength.
From the discussion above it is easy for a practitioner of the art, to see that the optimal base station location is that location from which the radio frequency coverage over the cell's geographical area is above a minimum acceptable level. Also, the selected location should work in conjunction with the surrounding cells served by the other base station sites within the constraints of the wireless technology at hand.
Not only is the location of the base station site very important, so is it's height (height of its antennas) as well as the physical placement of these antennas. For these reasons various methodologies and sophisticated software tools have been developed to assist in optimal base station site placement.
These tools are typically referred to as propagation prediction tools. They use terrain and even building databases to represent the topography in a given geographical area to a high degree of accuracy. This information is used to construct propagation path profiles, and apply the physical principles of radio wave propagation to predict the most likely average propagation path loss given the location of the base station, height and type of antenna at the base station, and the location and height of the receiving mobile device.
These programs calculate the predicted signal strength at a series of points blanketing the entire geographical area to produce detailed maps of radio frequency coverage strength and interference. The programs process various input and output layers. The inputs include terrain, land-use morphology, traffic or population data, and base station site data such as locations, antenna heights, types and orientations, as well as information that define what type of propagation model to use. Outputs include propagation signal strength and derived results, usually in the form of maps of coverage, interference, and relationships to other surrounding cells.
The propagation models used for prediction in these tools employ various degrees of abstraction to model the effects of reflection, diffraction, and scattering of the radio signals. Most will allow model parameter adjustments or generation of new models. Most have utilities to import radio survey data, perform statistical analysis, and implement changes to the models parameters to fit the data collected from the radio surveys. This process is called model tuning and is often preformed to adjust empirically derived models such as Okumura-Hata, COST-231-Hata and the Walfish-Ikegami model to name a few.
These empirical models are very widely used and accepted in the industry and are little more than mathematical curves fitted to radio survey data. This type of model is sufficiently accurate only if radio survey data is gathered for a sufficient number of representative sites and used to adjust the model for that particular geographical area and its representative morphologies.
As can be seen the exact prediction of signal levels from physical principles alone is impractical in real world situations. Radio surveys are thus much more reliable than predictions. These surveys are also necessary to tune the propagation models used.
Quite often a radio survey is required for any specific base-station site candidate before the space is leased or actual construction occurs. In most cases multiple candidates must be analyzed and perhaps surveyed using a test transmitter before any decision is taken. However, all these radio surveys are physically much more difficult, time-consuming and costly when compared to predictions.
Several manufactures market commercial tools for radio survey measurement data collection suitable for various mobile communication standards and radio frequency bands.